Trees Contribute to Stormwater Management

By Cindy Gilberg

Trees are the original multi-taskers, providing a variety of benefits that go beyond aesthetics. The value of trees in our communities includes the shade they provide, their function as air filters, the carbon dioxide they absorb and the oxygen they release. These benefits can be measured in dollars—quantitatively showing that perhaps money can grow on trees. In addition to the above benefits, the role trees play in stormwater management is significant. Environmental concerns about water pollution issues focus on reducing the quantity of stormwater runoff as one of the primary methods for improving water quality.

Much of the natural landscape that once absorbed rainfall has been replaced by the impervious surfaces of development—parking lots, roadways and rooftops. When trees are included in a landscape with other stormwater features such as rain gardens, bioswales and permeable pavement, a substantial reduction in stormwater runoff can be realized. Leaves and branches of trees intercept rainfall, slowing it down as it falls to the ground. Water evaporates from the large amount of surface area of a tree and its leafy canopy. Tree roots contribute to soil stabilization and make the soil more porous, allowing it to become like a “sponge” that readily absorbs rainfall rather than letting it run off the surface. The growth of a tree depends on water for growth – the larger the tree, the more water it uses. All of this adds up to a large percent of stormwater that is absorbed by trees and soil rather than being sent into regional stormwater systems.

Trees native to our region are quite hardy and well-adapted to our soils and climate. Be sure to note the cultural requirements of each species to help determine where in your landscape they should be planted. Some prefer a dry soil, such as the chinkapin oak (Quercus muhlenbergii), post oak (Quercus stellata) and serviceberry (Amelanchier arborescens). Other native tree species are tolerant of moist soil and aid in soaking up excess water in low areas that become saturated after storms. Bald cypress (Taxodium distichum), swamp white oak (Quercus bicolor) and black gum (Nyssa sylvatica) are prime examples of this group.

To figure the value of trees you may want to include in the landscape and for those that already exist, try some of the tree benefit calculators available online (such as www.itreetools.org or www.treebenefits.com ). According to one stormwater benefit calculator, a mature bald cypress can absorb almost 8000 gallons per year and a mature swamp white oak of the same size can absorb about 11,000 gallons per year.

An educational and fun activity to participate in locally to learn more about the benefits of trees is the Deer Creek Watershed, Webster-Kirkwood Mini Tree Hunt in October ( visit www.deercreekalliance.org for information, a map and online form). This tree hunt, sponsored by Missouri Botanical Garden’s Deer Creek Watershed Alliance and partners, including Missouri Department of Natural Resources, Sugar Creek Gardens, and the Cities of Kirkwood and Webster Groves, focuses on finding and identifying trees while learning about trees and how they contribute to stormwater management. Completed Tree Hunt forms will be entered in a drawing to win a $100 gift certificate from Sugar Creek Gardens.

Assess your landscape and determine how you can help be part of the stormwater solution – plant trees, rain gardens and use permeable pavement where possible. Drop by drop, it adds up to an improvement in water quality that benefits the entire community and region.

NOTE: The Deer Creek Watershed Alliance is funded by project partners, the Mabel Dorn Reeder Foundation, and the US EPA Region 7 through the Department of Natural Resources (subgrant number G09-NPS-13), under Section 319 of the Clean Water Act.

Cindy Gilberg is a Missouri native and horticulturist whose work includes design and consulting, teaching and writing. Much of her work focuses on native plants, habitat gardens and rain gardens. Cindy’s projects include work at Shaw Nature Reserve and its Native Plant School, the Shaw Professional Landscape Series and the Deer Creek Watershed Alliance. Cindy can be contacted at 314-630-1004; cindy.gilberg@gmail.com.